Almost certainly:

Johnstone, John Gordon, Sergeant, 42440, Canadian Infantry
Born Nottingham, England
Next of kin: Mrs. J. Johnstone, Fairmead, Risley, Derby, England
Date of birth 28th April 1888
Farmer
Enlisted Minnedosa, Manitoba, 18th December 1914

Aged 26 years 8 months
5 foot 6.5 inches in height
Clear complexion
Blue eyes
Fair hair
Presbyterian
Two scars on centre of nose

Information from attestation paper here:
http://www.collectionscanada.gc.ca/databases/cef/001042-119.01-e.php?&id_nbr=337113&interval=20&&PHPSESSID=6radh9am0irej9v45ed2hltsp2

However, there is no apparent record of Sergeant Johnstone having died during the war.

1911 Census:
John Gordon Johnstone was a 22 year old solicitor's articled clerk, residing at 14, Mapperley Road, Nottingham. He was the son of John Johnstone, a solicitor, and Ada Johnstone.
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